It is known that tungsten carbide WC is much used as an extrahard material, namely for the manufacturing of machining tools such as cemented carbide tools for cutting metals and drilling rocks and minerals. It has been established that some of the desirable properties of WC, such as for instance its resistance to rupture and to developing cracks under moderate stress are related, at least partly, to its hexagonal crystal structure. Further this carbide is very hard and resistant under hot conditions and its wettability by cementing bonding metals, such as Co, Ni and Fe, is excellent.
However tungsten is heavy and expensive and, for economical reasons, it is desirable to replace it by a lighter and more abundant metal having similar properties. Molybdenum is one such metal: indeed, molybdenum monocarbide MoC has a hexagonal crystal structure identical with that of WC and is also very hard. Unfortunately, it is not stable above about 1200.degree. C. (whereas WC retains its strength to temperatures up to about 2700.degree. C.) which prevents it from being used in all applications suited to WC.
It has been shown, however that solid solutions of tungsten and molybdenum monocarbides, that is materials of the formula Mo.sub.x W.sub.1-x C defined hereinabove, possess excellent physical properties which practically resemble those of WC, even with proportions of the molybdenum carbide as low as 1%. Such materials are therefore much advantageous, as compared to pure WC, with regard to lightness and price since the density of MoC is only 9 (as compared with 15.7 for WC) and molybdenum is a relatively abundant and cheap metal. Further, the heat resistance of the solid solution and decomposition temperature thereof, in between that of MoC and WC, is directly related to the magnitude of the W/Mo ratio. However, in order to exploit these advantages, the method of manufacturing such solid solution should be also economical.
There already exists some methods to fabricate such solid solutions or, more specifically, such solid solutions with, in addition, significant proportions of iron-group metals such as Co, Ni and Fe. It has indeed been found that such metals promote the formation of materials consisting of carbides of hexagonal structure and of the formula Mo.sub.x W.sub.1-x C wherein x is the same as in the above-given definition.
Thus, there was obtained a hexagonal carbide of formula Mo.sub.0.44 W.sub.0.56 C in admixture with cobalt by heating together WC, Mo.sub.2 C, and C with 4.8% of Co at 2000.degree. C. (Z. Anorg. Chem. 262 (1950), 212-217). In this respect, the Applicant has found that when mixtures of compressed powders containing WC, Mo and C or W, Mo and C were reacted in the presence of 2.5-10% Co by heating 4 hrs to 1200.degree.-2000.degree. C. (the temperature being dependent on the W/Mo ratio) under an inert atmosphere, there were also obtained materials containing mainly, as the carbide phase, Mo.sub.x W.sub.1-x C in which 0&lt;x&lt;0.8.
According to another method, mixtures of sintered powders containing, in atom-percent, about 41% Mo, 57% C and 2% Co were fused together with variable proportions of WC and the product was annealed 300 hrs at 1200.degree. C. under 10.sup.-6 Torr. There was thus obtained a series of materials containing mainly the carbide Mo.sub.x W.sub.1-x C where x can reach the value of 0.88 (Monatsh. Chem. 107 (1976), 1167-1176).
According to another process (German Patent Application DOS No. 2.623.990) concerning the preparation of solid solutions of WC in MoC containing iron-group metals, a mixture of graphite, metallic W and Mo in desired proportions, or a corresponding mixture of Mo.sub.2 C, WC and graphite, together with 0.5-1% of Ni or Co is heated to a temperature high enough to promote the formation of high-temperature stable phases. In this process, the temperatures are chosen as follows: Above 1975.degree. C. for inducing the formation of a solid solution of cubic carbides WC.sub.1-y and MoC.sub.1-y (y being defined in the recited reference and being called x in this reference); above 1680.degree. C. for inducing the formation of a solid solution of pseudo-cubic carbides W.sub.3 C.sub.2 and Mo.sub.3 C.sub.2. Then, the temperature is lowered to the range where the solid solution of the desired monocarbides is stable and maintained in this range for the time required to form the desired solution of hexagonal crystal structure.
However, these methods of the prior-art all have at least one of the following drawbacks: the need for a high temperature premelting of the ingredients, or the need for a preformation of the carbide phases at high temperature.
Moreover, having an iron-group metal present together with the desired extra-hard material is not always desirable in regard to further uses of the product and it may be wanted to maintain the level of such added metal as low as possible.
It will also be noted that, according to the prior-art, (Planseeber. fur Pulver Metallurgie 4 (1956), 2-6), it is recognized that solid solutions of hexagonal carbide Mo.sub.x W.sub.1-x C do not form easily when there are used, as starting substances, mixtures of C, W and Mo in the form of separate distinct phases. Thus, if graphite mixed with Mo metal (or carbide Mo.sub.2 C) and W metal (or carbide WC) is heated to 1700.degree. C., a solid solution of hexagonal type monocarbides does not form but, instead, there are formed two separate phases: a monocarbide WC and a carbide of formula (Mo,W).sub.2 C.